Hydraulic fracturing and fracture-acidizing are techniques commonly utilized to stimulate the production of oil and gas from subterranean formations of low permeability. In such treatments, fracturing fluids are introduced into the subterranean formation under sufficient pressure and having sufficient viscosity to create cracks or fractures in the formation and to also propagate these fractures out into the formation. The fracture fluids may contain entrained proppants, such as sand or sintered bauxite, so that as the fracture fluid seeps into the formation or is backflowed out from the fractures, the fractures close upon the proppants to maintain the fractures in an open state for increased permeability.
In using certain fracture fluids, such as high viscosity aqueous gels, water-hydrocarbon emulsions, or oil-based fluids, the high viscosity of these fracturing fluids should be maintained while the fractures are being created and propagated, as well as to aid in transporting the proppants to the farthest reaches of the fractures. After the proppants have been trapped in the fractures, however, it is desirable that the viscosity of the fracture fluids are quickly reduced to allow the fluids to flow back through the fractures, around the proppants and back into the wellbore. Chemicals utilized to reduce the viscosity of fracturing fluids are commonly called “breakers” or “breaker fluids” and are introduced into the fractures to act upon the fracturing fluids. The breakers, however, may be difficult to control. For example, when the breakers are introduced with the fracture fluid, they may immediately begin to reduce the viscosity of the fracture fluid before the fractures are able to form and/or the proppants are deposited. Thus, the breakers may break down the fracture fluids prematurely or at an inappropriate time.